Doped polyaniline solutions

ABSTRACT

Acid-doped, polyaniline-based polymers are formed into fibers, films, and coatings with a solvent of N-ethylpyrrolidone. Such a solvent system is particularly useful for the formation of a solid electrolyte on a capacitive element.

This application is a divisional of Ser. No. 08/962,059, filed Oct. 31,1997 now U.S. Pat. No. 5,853,794.

FIELD OF THE INVENTION

The invention relates to doped polyaniline solutions, solvent systemsused therein, and conductive articles formed therefrom.

BACKGROUND OF THE INVENTION

Acid-doped polyaniline is finding an increasing level of interest as asolid electrolyte for capacitors and other electronics devices. Thedopants are selected from a relatively small group of acids that enhancesolubility in organic solvents with a low dielectric constant, e.g., adielectric constant of less than about 17. Acid-doped polyanilinepolymers are generally considered to have no practical solubility insolvents with a dielectric constant of greater than about 17. See U.S.Pat. No. 5,567,356. The most preferred polyaniline dopant is dinonylnaphthalene sulfonic acid (DNSA).

A commercially available solution contains DNSA-doped polyaniline in asolvent mixture containing xylene, ethylene glycol monobutyl ether.Unfortunately, this solvent system is characterized by toxicity, anobjectionable odor, and a solvency for inorganic salts that issufficiently low to preclude use of the solvent for co-depositing suchsalts with the polymer.

It would be useful to have a solvent system that exhibited a low vaporpressure and a low toxicity.

It would be even more beneficial to have a solvent system that also hada relatively high boiling point and a relatively high dielectricconstant. The higher boiling point would facilitate the use and handlingof the solvent in a commercial setting. A higher dielectric constantwould increase the conductivity to a point that was similar to systemsusing polar solvent salt systems and would be useful for some types ofcapacitors where an electrically conductive residue remained aftersolvent evaporation.

Enhanced systems for forming polyaniline-based polymer films andcoatings would be particularly useful in the manufacture of capacitors.In such articles, one or more coatings of electrically conductivepolyaniline-based polymer can be used as a solid electrolyte between thedielectric oxide layer and the electrodes. It would be useful to have apolyaniline-based polymer dissolution system that did not pose theproblems and limitations of the former xylene solvent systems.

SUMMARY OF THE INVENTION

It is an objective of the invention to provide a process for depositinga coating or film of polyaniline from a solution that uses a solventcharacterized by a lower toxicity, lower vapor pressure, higher solvencyfor inorganic salts, higher boiling point, and higher dielectricconstant than xylene-based solvent systems used previously.

In accordance with this and other objectives of the invention that willbecome apparent from the description herein, a process, coating, andcoated article according to the invention are based on the use of asolvent system that use N-ethyl-pyrrolidone as the solvent foracid-doped polyaniline-based polymers. This solvent is characterized bythe desired lower toxicity, lower vapor pressure, higher solvency forinorganic salts, higher boiling point, and higher dielectric constantthan xylene-based solvent systems.

DETAILED DESCRIPTION

Acid-doped polayaniline-based polymer is dissolved inN-ethyl-pyrrolidone solvent and used to form conductive articles. Sucharticles include fibers, films, coatings (particularly coatings fordissipating static electricity), coated articles, batteries,electrolytic sensors, and capacitive elements. One coated article ofparticular interest is a capacitive element that uses an acid-dopedpolyaniline-based polymer as a solid or liquid electrolyte. The anodebody of such a capacitor is preferably made of a valve metal likealuminum or tantalum, with tantalum being generally more preferredbetween the two.

The polyaniline-based polymers applicable for the present invention aregenerally described in U.S. Pat. No. 5,069,820 the disclosure of whichis herein incorporated by reference. The '820 patent describeselectrically conductive, polyaniline-based polymers having the followinggeneral formula: ##STR1## wherein: n is an integer from 0 to 5;

m is an integer from 0 to 5 with the proviso that the sum of n and m isequal to 5;

R2 and R4 are the same or different and are hydrogen or alkyl of 1-10carbon atoms;

R3 is the same or different and is selected from alkyl, alkenyl, alkoxy,cycloalkoxy, cycloalkenyl, alkanoyl, alkylthio; alkylamino, aryloxy,alkylthioalkyl, alkylaryl, arylalkyl, amino, dialkylamino, aryl,aryloxyalkyl, alkylsulfinylakyl, alkylsulfonyl, arylsulfonyl, carboxylicacid, halogen, cyano, sulonic acid, nitro, alkylsilane, or alkylsubstituted with one or more of sulonic acid, carboxylic acid, halo,nitro, cyano, or epoxy moieties; or any two R3 groups taken together mayform an alkylene or alkylene chain completing a 3, 4, 5, 6, or7-membered aromatic or acyclic ring that may include one or moredivalent nitrogen, sulfur, sulfinyl, ester, carbonyl, sulfonyl, oroxygen atoms; R3 is an aliphatic moiety having repeat units of either ofthe formula:

    --(OCH.sub.2 CH.sub.2).sub.q O--or--(OCH.sub.2 CH(CH.sub.3)).sub.q O--

wherein q is a positive whole number.

The acid dopants used for the polyaniline-based polymers are generallyselected from anions of sulfonic acids (e.g., dinonyl naphthalenesulfonic acid (DNSA), toluenesulfonic acid, dodecylbenzine sulfonicacid, camphor sulfonic acid, allylsulfonic acid, 1-propanesulfonic acid,1-butananesulfonic acid, 1-hexanesulfonic acid, 1-heptanesulfonic acid,benzenesulfonic acid, styrenesulfonic acid, naphthalenesulfonic acid,including homologs and analogs thereof), and carboxylic acids (e.g.,acetic acid and oxalic acid). The preferred organic sulfonic aciddopants includes toluenesulfonic acid, dodecylbenzine sulfonic acid, andcamphor sulfonic acid.

The primary solvent useful in the present invention isN-ethylpyrrolidone. This solvent is characterized by a high dielectricconstant (about 28), high solution conductivity, the ability to wetsubstrates that are otherwise difficult to wet, and a freezing point ofabout -70° C. The high dielectric constant suggests that the solventshould be able to dissolve salts for co-deposition with thepolyaniline-based polymer.

The solvent of the invention allows polyaniline-based polymer solutionsto be made at a variety of concentrations from 0.01-35 wt % withstirring and heating of 80°-85° C. at the higher concentrations. Themore commercially useful concentrations for thorough impregnation andcoating of porous substrates is a solution having from about 10 wt % toabout 20 wt % polyaniline-based polymer. Such solution concentrationsare comparable to those commercially available with xylene-based solventsystems.

The polyaniline polymer solution of the present invention isparticularly suitable in the manufacture of capacitive elements that usean electrically conductive, acid-doped polyaniline polymer as anelectrolyte. Such capacitive elements are made from valve metal powdersthat are anodized to form a dielectric layer on the surface of the anodebody, coated with an electrically conductive polymer to form anelectrolyte layer, reformed, coated/reformed until the desiredelectrolyte thickness is achieved, and finished. Liquid electrolytes aregenerally preferred for high voltage capacitive elements.

The valve metals from which the capacitive elements are formedpreferably are made of materials that form an insulating film when thebody is positively charged. When the body is negatively charged, thefilm will conduct. Suitable materials include the Group IV and V metals(particularly niobium, tantalum, zirconium, and titanium) and aluminum.When powdered, appropriate powder sizes are within the range of 0.05 to50 microns. These powders are pressed with or without a binder to form agreen anode body having a density of about 30-70% theoretical density.The green body is then sintered at a temperature within the range fromabout 1200° C. to about 1800° C. Aluminum is preferably used in the formof a foil or etched foil that is either rolled or stacked.

The anode is then "anodized" by suspending the sintered body in anelectrolyte solution at a formation voltage of 3-4 times the ratedvoltage of the element. For example, a typical part rated at 10 voltswould be formed at 30-40 volts, usually 35 volts. Suitable electrolytesolutions include phosphoric acid or ammonium nitrate in water with orwithout thickening agents, solvents, co-solvents, surfactants, or otherconventional additives.

Once anodized, the anode is coated with one or more layers of anelectrically conductive, acid-doped polyaniline-based polymer byimmersing the capacitive element in a solution containing the polymer inthe solvent. The coated element is then heated to drive off the solvent.Suitable heating temperatures are within the range from about 35° C. toabout 120° C.

The polymer-coated capacitive element is then "reformed" by immersingthe element in an acidic reforming solution. After heating, there may beresidual monomer or by-product materials that are undesirable in thefinal capacitive element. Such materials are readily removed by washingwith water, solvents, and/or surfactants. Preferred washing agentsinclude methanol or acetone.

The thickness of the electrolyte layer can be increased by repeated theabove process steps until an adequate thickness is achieved. In general,the polymeric coating can be built up with 1-20 repetitions of theimpregnation, heating, and washing steps.

The reformed capacitive element is then finished to make a stock part.Finishing would typically entail an outer coating of the undoped solidelectrolyte polymer, imprinting the element with an electrode pattern,sealing the unit in a nonconductive material, e.g., epoxy, and forming amultielement assembly (if desired).

EXAMPLES

Example 1

A 25% solution containing 10 grams of DNSA-doped polyaniline is preparedby dissolving polyaniline in an 80° C. solution of N.E.P.™(International Specialty Chemicals Corp.), a commercially availableproduct of N-ethyl-2-pyrrolidone. The solution has a dielectric constantof 28.1. Resistivity measurements were made at 1 kHz with a Y.S.I.conductivity cell (cell constant=1.0) and are reported in Table 1.

                  TABLE 1    ______________________________________    TEMPERATURE (° C.)                    RESISTIVITY (OHM-CM)    ______________________________________    -35             8350    20              2070    80              1000    ______________________________________

The conductivity of this solution is similar to that of a polar solventsalt solution.

We claim:
 1. A liquid solution containing:N-ethylpyrrolidone and anacid-doped polyaniline-based polymer.
 2. A liquid solution according toclaim 1 wherein said acid-doped polyaniline polymer is a sulfonicacid-doped polyaniline polymer.
 3. A liquid solution according to claim2 wherein said acid-doped polyaniline polymer is a dinonyl naphthalenesulfonic acid-doped polyaniline polymer.
 4. A liquid solution accordingto claim 1 containing 0.01-35 wt % of said polymer.
 5. A liquid solutionaccording to claim 4 wherein said polymer is dinonyl naphthalenesulfonic acid-doped polyaniline.
 6. A liquid solution according to claim4 containing 10-20 wt % of said polymer.
 7. A liquid solution accordingto claim 6 wherein said polymer is dinonyl naphthalene sulfonicacid-doped polyaniline.